The present invention relates to membrane modules utilizing innovative geometries of net-type feed spacers for improved performance in separations and spacer-fabrication methods therein. Such improved geometric configurations of net-type feed spacer sheets, which are essential components of spiral-wound membrane (SWM) modules, critically affect the performance of SWM modules, which are employed in water purification, water desalination, and other liquid-separation processes.
Pressure-driven membrane technology is widely applied for the purpose of water purification from dissolved or suspended salts, colloids, organic molecules, and other undesirable species. Such membrane technology has also been applied to other types of liquid purification in which the undesirable species are removed by taking advantage of physicochemical properties of particular membranes. According to the physicochemical characteristics (e.g., size, polarity, charge) of the species which need to be separated, various types of membranes are used which (in order of increasing pore size) are commonly classified as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) membranes. RO and NF membranes are usually employed as the final step in water desalination for removal of ionic and other dissolved species. Whereas, UF and MF membranes are utilized for fluid pretreatment and removal of the largest proportion of dispersed colloids, macro-molecules, and other (relatively large-size) species in processes of liquid purification or recovery of valuable products, either in the retentate stream or the liquid permeate.
In another category of new membrane methods (referred to as osmotic separations), the osmotic pressure difference between the treated aqueous (or other) solution and another liquid is exploited to separate the two liquids using a semi-permeable membrane. Employing the osmotic pressure difference between the two liquids and the concentration of solutions as the driving force, various separations can be achieved without applying pressure.
To implement membrane-based water treatment and similar separation processes, special modules are commonly used, which are comprised of flat-sheet membranes. For the typical case of water treatment and desalination with RO and NF membranes, the so-called spiral wound membrane (SWM) module is employed (henceforth referred to as an “SWM module” or “SWM element”).
Referring to the drawings, FIG. 1 is a simplified perspective diagram of a typical SWM module having a standard mesh-type spacer used in water desalination and membrane water treatment/separation, according to the prior art. Two membrane sheets 2, which are closed (or glued) along three sides/edges with the fourth side/edge open (enveloping a fabric-type insert to facilitate permeate flow), and properly attached to a central perforated tube 4, form an “envelope.” Central perforated tube 4 receives the out-flowing permeate along a transverse direction 3. Feed liquid flows axially (as in feed flow 5) through narrow channels which are formed from a usually-polymeric, mesh-type spacer 1 placed in between two adjacent envelopes. Due to the applied pressure on the feed liquid-side (or/and because of the osmotic pressure difference), permeate passes through semi-permeable membrane 2, and flows in transverse direction 3 with respect to axial feed flow 5, through the permeate channel toward central perforated tube 4 along the axis of the SWM module from where the permeate is removed as the purified product. As the feed liquid flows axially (as in feed flow 5) to the exit of the SWM element, permeate removal causes a reduction in volume, and the concentration of rejected salts and foulants tend to increase. Consequently, the purified product is often called “concentrate,” and possesses increased osmotic pressure in comparison to the initial feed liquid to be desalinated.
Two basic problems related to the aforementioned membrane operation, which are significantly affected by the feed-spacer geometry, are concentration polarization (CP) and membrane fouling (MF). CP is the result of the development of the concentration boundary-layer (from the rejected salts) at the membrane surface in which the concentration of ionic species is significantly greater than in the liquid bulk; therefore, the osmotic pressure at the membrane surface is much greater than that corresponding to the fluid bulk concentration. Such an increase in osmotic pressure at the membrane surface reduces the effective trans-membrane pressure, resulting in a negative impact on separation performance (i.e., leading to reduced permeate flux), as well as on energy expenditure and the economics of the membrane process (see R. Semiat, Environ. Sci. Technol., 42(22), 8193-8201 (2008)).
It is noted that the effective trans-membrane pressure is obtained from the pressure difference between the retentate and permeate channels minus the osmotic pressure difference between those channels. CP also results in reduced salt rejection of the membrane, which negatively affects the permeate quality. The problem of MF by the rejected species (e.g., colloids and organic molecules) is due to the formation of a layer comprising these species, which is rather firmly attached to the membrane surface. Such a layer tends to significantly increase the resistance to the permeate flow, enhance CP, and reduce the effective transmembrane pressure, leading to overall deterioration of membrane separation performance—both qualitatively and quantitatively.
The inevitable use of spacers for the creation of the feed-water channels, while possibly intensifying in part the aforementioned pressure-drop and fouling, could also serve to mitigate CP and MF as outlined in research by Koutsou et al. (see J. Membrane Sci., 291, 53-69 (2007)). The work of Koutsou et al. has proven that, even at relatively low flow rates, spacers can promote fluid-dynamic instabilities and turbulence development, as well as increased flow shear-stresses at the membrane surface, which lead to mitigation of CP and MF phenomena. At the same time, spacers are responsible for a pressure-drop increase at the feed-water channels in comparison to the ideal case of spacer-free channel flow.
Presently, spacers of the net-type configuration are widely-used, and are generally quite thin, with thickness (depending on the application of the membrane module) varying from 0.6 mm to 1.2 mm (see Schock and Miguel in Desalination, 64, 339-352 (1987)). Returning to FIG. 1, spacers 1 are placed in between neighboring envelopes, which are formed by two membrane sheets 2 (sealed together on three sides), creating the necessary gap for the feed-water flow (axial feed flow 5). Moreover, spacers, depending on their detailed geometrical characteristics, tend to create flow instability (e.g., generating vortices), and increase flow shear-stresses at the membrane surface, resulting in reduction of CP and possibly mitigation of MF which negatively affect the separation performance. Therefore, with the appropriate geometrical spacer-configuration, it is possible to improve overall SWM-module performance through the aforementioned mechanisms and increased mass-transfer coefficients at the membrane surface.
The inherent disadvantages of feed-spacers, which are currently used in commercial membrane modules, are due to their presence in the narrow feed-water channels, which promote the creation of dead-flow zones (which enhance fouling), and cause enhancement of the pressure drop (i.e., energy consumption), resulting in increased process operating cost. Therefore, in the past two decades (see Fimbres-Weihs and Wiley in J. Membrane Sci., 326, 234-251 (2009)), the geometrical characteristics of such spacers have been the subject of significant research, aiming at optimization of their geometry, leading to improved SWM-module performance (through increased shear stresses and mass-transfer coefficients) with the lowest possible pressure drop.
FIG. 2 is a simplified perspective diagram of a typical net-type spacer, according to the prior art. Typically, common net-type spacers are formed by two planar rows of parallel (nearly cylindrical) filaments 6 (having diameters varying between ˜0.35 mm and ˜0.60 mm), which intersect with each other at a given angle, x. In such a configuration, each row of cylindrical filaments 6 (whose axes of symmetry are on the same plane) touches one membrane sheet of the respective feed-water channel. Thus, such a geometry is characterized by flow constriction 7 (i.e., narrow open areas) which are formed between the cylindrical filaments (of one row) and the opposite membrane surface, and by contact lines 8 of the same filaments with the other membrane surface.
Detailed theoretical and experimental research results (see Koutsou et al. cited above and Koutsou et al. in J. Membrane Sci., 326, 234-251 (2009)) show that, in narrow open areas (i.e., flow constrictions 7) between spacer filaments and the membrane surface, the mass-transfer coefficients are significantly increased, which positively affects the performance of the membrane modules. However, at (and near) the contact lines, the corresponding coefficients are almost zero, leading to enhancement of the undesirable phenomena of CP and MF. The latter tend to degrade SWM-module performance, and reduce the membrane lifetime due to frequent chemical cleaning, which usually damages the membrane's active surface.
For optimization of spacer geometric characteristics, care should be taken to balance the counteracting requirements of pressure-drop minimization (which is achieved with thicker spacers, and thus, greater gaps and lower fluid velocities) versus maximization of shear stresses and mass-transfer coefficients (which are achieved by increased fluid velocities and/or thinner spacers). In addition, the work of Koutsou et al. has theoretically and experimentally proven that the detailed spacer geometrical configuration, and its orientation with respect to the direction of the incoming mean flow, affect pressure drop and other significant parameters (i.e., flow shear-stresses at the membrane surface and mass-transfer coefficients). Both of these parameters in turn significantly affect CP and MF.
Presently-used, net-type spacers are fabricated using methods described in detail in U.S. Pat. Nos. 3,700,521; 3,957,565; and 3,067,084. Such production methods are based on extrusion of polymeric material through two concentric circular arrays of dies, arranged on two concentric circles of different diameters. Polymeric material extrusion in the form of filaments takes place at different angle from each circular array of dies so that when the filaments of one array (still in the form of a melt) touch the filaments from the other array, the filaments tend to adhere to each other, forming a tubular biplanar net with the desirable “crossing angle” x of the filaments. In the automatic production process, the ensuing filament, cooling under tension of the net, leads to the final filament-shape formation. Cutting such a tubular net leads to the final planar form of the marketable spacer net.
It would be desirable to have membrane modules utilizing innovative geometries of net-type feed spacers for improved performance in separations and spacer-fabrication methods thereof. Such modules and methods would, inter alia, overcome the various limitations mentioned above.